1. Field of the Invention
The present invention relates to a cooling system for a wet clutch in a motor vehicle.
2. Description of the Prior Art
In general, a friction clutch for a motor vehicle is enclosed in a bell-shaped housing which is secured to the engine block. The bell housing also encloses a flywheel which is attached to the output end of the engine crankshaft for rotation therewith. The flywheel has a friction face or driving surface which can be engaged by friction linings mounted on a clutch disk. The clutch disk is attached to an output shaft which may be connected to a set of gears in a transmission. A pressure plate is forced against the clutch disk by a plurality of clutch springs acting against a clutch cover attached to the flywheel. The clutch disk is forced into engagement with the driving surface of the flywheel such that the rotational movement of the engine is transmitted to the output shaft.
Typically, the bell housing contains a fluid, such as air or oil, in which the clutch operates. The rotation of the clutch disk, the pressure plate and the clutch cover tends to expel the fluid from the vicinity of these elements under the influence of centrifugal force. Heat generated during the slipping of the clutch elements which are not in full engagement is carred by the fluid towards the walls of the bell housing through apertures in the cover plate. Generally, heat dissipation by conduction through the walls of the bell housing is slow. If the clutch slips for any appreciable amount of time, the temperature of the clutch elements and the surrounding fluid will increase substantially which will tend to shorten the clutch life. Cooling fins formed on the exterior surface of the bell housing will increase the rate of heat dissipation, but do not provide a significant reduction in clutch operating temperatures under prolonged slip conditions.
One method of increasing heat dissipation is to pass the fluid through an external heat exchanger. In one form of a friction clutch where the cooling fluid is air, the bell housing is provided with two ports, one of which is located nearer to the radial periphery of the clutch than the other. The ports are connected to a heat exchange means external of the bell housing which may be a hollow casing mounted in the air stream created by movement of the vehicle. The casting may also have fins formed on its external surfaces for increased heat dissipation. Since the rotation of the clutch elements creates a zone of relatively high pressure in the vicinity of the nearer port, whereas the pressure in the bell housing in the vicinity of the other port remains at a lower value, a pressure differential is created to generate a flow of air through the heat exchange means such that the heated air is carried away from the clutch and is cooled in the heat exchange means before being returned to the bell housing. The nearer port may be inclined in the direction of rotation of the clutch so that the tangential components of the induced air flow in the vicinity of the clutch assist in creating the flow through the heat exchange means. Furthermore, the other port may be inclined opposite to the direction of rotation of the clutch so that the tangential components of the induced air flow in the vicinity of the clutch assist in the return of the air to the bell housing. Such a heat exchange means is disclosed in U.S. Pat. No. 3,661,238, issued May 9, 1972 to David A. Davies.
Where the fluid is oil, another method of increasing heat dissipation is to provide scoops attached to the clutch elements to dip into the oil stored in the sump of the bell housing. These scoops may be in the form of tubes having an inlet inclined in the direction of rotation of the clutch and an outlet in the vicinity of the engagement surface of the clutch disk. Alternatively, the scoops may be formed as integral hook-shaped diverters on the periphery of the clutch disk. Each diverter has a finger portion tapered in a direction opposite the direction of rotation of the clutch elements and has a curved ramp on its inner edge which functions to divert the oil radially inwardly of the disk into a diagonal slot from which the oil feeds into grooves in friction facings on the disk. The oil is retained between the flywheel and the clutch cover and tends to rotate therewith. Until full engagement is achieved, the clutch disk will rotate slower than the flywheel and clutch cover with an opposite relative direction of rotation to render the diverts effective. However, the difference in speeds and, therefore, the quality of cooling oil diverted is the least at low slip conditions where the highest amount of heat is generated. Therefore, this type of cooling system is least effective under prolonged low slip conditions when increased cooling is required. A wet clutch having hook-shaped diverters is disclosed in U.S. Pat. No. 3,897,860, issued Aug. 5, 1975 to Howard O. Borck, et al.